Drupal\media\Entity\Media::prepareSave should convert URL object metadata to string before saving

Problem/Motivation

Media bundles allow for source values to be mapped to fields on the bundle. The mapping is executed in Drupal\media\Entity\Media::prepareSave. There is an issue with how URLs are processed. Sometimes the metadata value is a string. Other times it is a \Drupal\Core\Url object. When the mapped value is saved, the assumption is that the value is a string. Take a look at Drupal\media\OEmbed\Resource::getAuthorUrl. This method calls Drupal\Core\URL::fromUri, which expects to receive a string. If it receives a URL object, then the result is a fatal error.

Steps to reproduce

1. Create a 'Remote Video' media bundle
2. Create an 'Author URL' field (Link field
3. Map this field to 'The URL of the author/owner'
4. Attempt to add a YouTube video as a Media entity
5. Note the following error for a Link field: Recoverable fatal error: Object of class Drupal\\Core\\Url could not be converted to string in /app/web/core/lib/Drupal/Core/Url.php on line 281

If the mapped field is a text field, then then the result is a different fatal error:

Or the following error for a Text field: <code> Uncaught PHP Exception Drupal\\Core\\Entity\\EntityStorageException: "Expected argument of type "string", "Drupal\\Core\\Url" given" at /app/web/core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorage.php line 847

Proposed resolution

Drupal\media\Entity\Media::prepareSave can check if a particular metadata is a URL object. If so, convert the object to a string before saving.

Remaining tasks

Community review.

User interface changes

None.

API changes

None.

Data model changes

None.

Release notes snippet

None.